1. Technical Field
The present invention relates to a fixing device and an image forming apparatus incorporating the same, and more particularly, to a fixing device that fixes a toner image in place with heat and pressure, and an image forming apparatus incorporating such a fixing device.
2. Background Art
In electrophotographic image formation, an image is formed by attracting toner particles to an electrostatic latent image on a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium to obtain a print output.
One specific type of the fixing device is a roller-based fixing device employing a pair of cylindrical fixing rollers, one being a fuser roller subjected to heating, and the other being a pressure roller disposed opposite the fuser roller. The pressure roller presses against the fuser roller to form a fixing nip therebetween, through which the recording sheet is conveyed. At the fixing nip, the fuser roller heats the incoming sheet to fuse and melt the toner particles, while the pressure roller presses the sheet against the fuser roller to cause the molten toner to set onto the sheet surface.
Another, more thermally efficient fixing device employs a flexible, endless fuser belt formed of material with a low heat capacity, subjected to heating and disposed opposite the pressure roller. Compared to the roller-based configuration, which involves a pair of fuser and pressure rollers both exhibiting a relatively high heat capacity, the belt-based fixing device can swiftly heat the fuser assembly to a desired operational temperature upon start-up.
To date, some fixing devices employ induction heaters (IH) to heat a fuser member, such as a belt or roller, through electromagnetic induction heating. Use of induction heaters, which generally exhibit higher heat transfer rates, and therefore can heat the fuser assembly more rapidly than conventional heaters such as halogen heaters, allows for fast effective heating performance in thermal fixing process.
Electromagnetic induction heating, although highly efficient in terms of heat generation, may not work properly as desired when applied to heating of a rotatable fuser member. That is, where the fuser member occasionally stops rotation while subjected to electromagnetic induction heating, an extraordinary high amount of heat is imparted to a limited portion of the fuser member facing the induction heater. Such localized, excessive heating cause variations in temperature distribution across the fuser member, which translates into improper amounts of heat exerted to a toner image from the fuser member, resulting in incomplete fixing performance.
To address this problem, several methods have been proposed to provide a fixing device with a heating control capability that controls heating of a rotatable fuser member, in particular, an endless belt, according to readings of a rotation sensor detecting rotation of the fuser member.
For example, one such method employs a rotatable fixing film formed into a cylindrical configuration. The fixing film has a plurality of detection marks disposed along a rotational, circumferential direction of the fixing film, to which an optical sensor is directed to detect movement of the fixing film in the circumferential direction based on displacement of the detection marks. Instead of an optical sensor, detection of the film movement may be implemented using a rotary encoder in combination with a detection roller disposed in contact with the fixing belt.
Another method employs an endless fuser belt entrained around multiple rollers, including a fuser roller disposed opposite a pressure roller via the belt, and a heat roller subjected to induction heating. The fuser belt has one or more perforations along a circumferential, rotational direction thereof, to which an optical sensor is directed to detect movement of the belt in the circumferential direction based on displacement of the perforations.
Still another method employs an endless fuser belt including a thermally conductive, heat-generating layer that generates heat when subjected to induction heating. The fuser belt has one or more detection portions that are relatively transparent to infrared radiation compared to the heat-generating layer along a circumferential, rotational direction thereof, to which an infrared sensor is directed to detect movement of the belt in the circumferential direction based on displacement of the detection portions.
Yet still another method employs an endless fuser belt entrained around multiple rollers, including a fuser roller disposed opposite a pressure roller via the belt, and a heat roller subjected to induction heating. A detection roller is disposed opposite the heat roller via the belt to rotate with the belt. A rotation detector is provided, including an optical rotary encoder formed of a notched disk and a photodetector directed to the notched disk to detect rotation of the detection roller indicating concurrent movement of the belt in the circumferential direction.